X-ray image converters utilizing luminescent materials have long been known, such as radiographic intensifier screens, fluoroscopic screens, and X-ray image intensifier tubes. For example, in issued U.S. Pat. No. 3,795,814 there is disclosed lanthanum and gadolinium oxyhalide luminescent materials activated with thulium ion as efficient materials to convert X-ray radiation to ultraviolet and visible light. Various image converter devices utilizing said luminescent materials are also described therein for conversion of the X-rays to blue color emission, including a multi-layer type X-ray screen construction for use with photographic film to record the light image. In more recent U.S. Pat. No. 3,996,472, also assigned to the present assignee, various rare earth oxyhalides coactivated with terbium and a second activator selected from zirconium and hafnium exhibit superior thermoluminescent behavior in radiation dosimeters when subjected to heat stimulation.
It has now become desirable to produce lower speed rare earth oxyhalide phosphors activated with thulium ion for X-ray screen application. In this product application, a modified phosphor material is desired exhibiting reduced quantum noise with speeds of about 4 times that of the conventional PAR screens. Known thulium activated lanthanum oxybromide phosphors exhibiting such reduced efficiency can be produced with lower thulium activator levels and further containing ytterbium ion. Such phosphor variation produces a broad range of scintillation intensities, however, with the number of scintillation events being distributed over a broad range of scintillation intensity values. This is undesirable because high quantum noise is produced giving rise to highly mottled film images. Lower concentrations of thulium activator ion are also apparently not uniformly distributed in the phosphor host lattice so that some portions of the lattice convert the absorbed X-rays more efficiently while other lattice regions convert the absorbed radiation less efficiently to light photons. The presence of the ytterbium ion also apparently does not alter this aspect of the light emission although having been found to reduce afterglow. Further modification of these phosphor materials to impart greater uniformity in the scintillation intensities would thereby provide significant improvement for the desired product application as well as for still other X-ray image converter devices.